The present invention relates to optical methods and devices for measuring the distance from an obstacle. In particular, the invention concerns optical devices of the type comprising:                a unit to emit radiation and to conform the irradiated beam,        an acquisition unit to acquire optical information relating to the image of the scene in front of the optical device, and        an electronic control unit to receive signals leaving the acquisition unit and to process them for the purpose of determining the distance between said optical device and an obstacle that is part of the scene in front of the optical device.        
Techniques of the optical type to measure the distance from an obstacle may be subdivided into passive and active techniques. Passive techniques exploit the existing environmental illumination, whereas active techniques illuminate the scene with a source of radiation.
The most widely used passive technique, above all in robotics and industrial automation, is the so-called stereo technique, based on the use of two video cameras that observe the same portion of the scene from different angles.
Active techniques are however undoubtedly more robust and better adapted to conditions of high background light, or those of poor visibility.
Among the active techniques, the triangulation technique has been proposed for numerous different applications, both with a single beam and with structured light, for example in the form of a blade of light, scanning in one or two dimensions and observed from a direction at an angle with regard to the direction of projection. Triangulation enables the processing complexity to be reduced, but there are some limits to the precision that can be achieved.
Both the stereo technique and triangulation, indeed, become less precise for large distances between sensor and obstacle, taking into account that the accuracy depends on the relative sensor-sensor or source-sensor distance. Furthermore, measurement depends on calibration of the system, with a high sensitivity with regard to mechanical adjustments; all triangulation systems are extremely sensitive to misalignment.
For other applications that require detection in the medium/long range, such as automobile-related applications, active ranging techniques have been used, based on infrared sources (laser), with multiple beams or in some cases with a scanning beam operating in one or two dimensions. Techniques have also been proposed that are based on measuring the (direct or indirect) flight time with pulsed sources. For example, in US2004233416 a device is described to produce a three-dimensional image representing the distance of obstacles. The image is focused on a matrix of CMOS sensors and the distance is calculated by measuring the back-reflected signal of packets of pulses sent, out of phase by a few tenths of a nanosecond. Lastly, techniques have been proposed based on the measurement of the degree of phase displacement between emitted signal and reflected signal, with modulated sources of the sinusoidal type. As an example, a method is described in US2004008394 to acquire and demodulate electromagnetic waves modulated from the temporal standpoint by means of two micro-optical elements capable of focusing the same portion of the scene onto two distinct sensitive areas. In this way it is possible to measure, for example, the phase of a modulated signal and apply this technique in distance measurement systems by calculating the flight time. The sequential nature of the measurements connected to the scanning, the need to work with high frequencies (pulse duration less than a microsecond) and the high degree of complexity of the system are the principal disadvantages of these techniques for vehicle applications, whereas the chief advantage with regard to triangulation techniques lies in the greater ease of installation and calibration, as well as the greater accuracy in distance measurement for long obstacle distances. Prototype 3D vision systems (ranging cameras) are also in the development phase, for robotics and security applications. In such systems, a matrix of photodetectors, with appropriate processing electronics integrated at the pixel level, performs two functions in parallel on an entire image, detecting the scene and measuring the distance of objects.